1. Field of the Invention
The present invention relates to a vinylidene chloride copolymer composition, more particularly, it relates to a vinylidene chloride copolymer composition which is produced by copolymerizing vinylidene chloride monomer and a comonomer under dissolving a rigid polymer in the vinylidene chloride monomer and which has excellent extrusion processability, and has the improved surface condition of the molded product.
2. Field of the Invention
It has been known that vinylidene chloride copolymer compositions comprising vinylidene chloride copolymer produced by copolymerizing 60 to 95 wt. parts of vinylidene chloride and 40 to 5 wt. parts of a comonomer (total 100 wt. parts) and 5 to 15 wt. parts of a modifier e.g. a plasticizer and a stabilizer, if necessary, 1 to 20 wt. parts of a modifier e.g. a rubber-like elastomer, are used for the extrusion molding to obtain molded products such as film, bottle etc. by blending, melting and extruding the composition by a screw type melt extruder.
The molded products have excellent tensile strength, flexibility and transparency and excellent gas barrier characteristic whereby they have been widely used as packages for food and others.
However, they have been used for various usages. In the packages for retort food, dry food and rottable food, the gas barrier characteristics of the molded products have not been satisfactory. In order to adapt the products to have high gas barrier characteristic, and to treat at high speed in the automatic packaging, printing and laminating machines, the compositions having superior physical characteristics have been required. Various methods have been proposed to satisfy the demands. However, the known compositions have certain disadvantage and the composition having both of excellent gas barrier characteristic and physical characteristics has not been known.
The gas barrier characteristic can be improved by decreasing the amounts of plasticizer and other additives. However, it has been known that the copolymers having more than 60 wt.% of vinylidene chloride component have each melting point which is near the decomposing temperature and when the content of the plasticizer is decreased to be substantially the component of the copolymer, the copolymer is easily thermally decomposed in the extrusion molding operation.
In order to improve the physical characteristics, a powdery additive such as an inorganic material e.g. metal oxides and hydroxides and an organic material e.g. polymers having high melting point is incorporated to form rough surface of the molded product or a lubricant having a melting point of higher than the room temperature such as higher fatty acids, modified higher fatty acids, paraffin and waxes or a crystalline material is incorporated to migrate it on the surface of the molded product or to impart the smoothness of the surface of the product.
When the powdery material is incorporated, there is the possibility to disadvantageously form pin-holes on the film product. When the lubricant or the crystalline material is incorporated, the additive causes adverse effect in the printing and laminating operation. When the film product is used for wrapping food, the additive is easily migrated to be disadvantageous from the viewpoint of hygiene, moreover, the extrusion processability is not stable and the extruding rate is decreased because of slippage in the extruder. Accordingly, the amount of the additive has been limited to be small and the physical characteristics have not been satisfactory by the addition of a single additive. In many cases, both of the powder and the lubricant or the crystalline materials have been incorporated to impart the balance of the improved physical characteristics and the deterioration of the other characteristics.